Recent advances in combustion technology have employed the use of high velocity gas injection into a combustion zone to carry out combustion with reduced nitrogen oxides (NO.sub.x) generation. Nozzles with relatively small diameters are employed in order to achieve the high velocities. The high gas velocities cause furnace gases to be aspirated or entrained into the high velocity gas which has a dampening effect on NO.sub.x generation.
A problem with high velocity gas injection into a combustion zone is that the furnace gases, which may comprise particulate matter and condensible vapors, cause the nozzles, which have small openings to begin with, to foul or corrode easily as the furnace gases are aspirated or entrained into the high velocity gas exiting the nozzle. The furnace gases also tend to be quite hot, on the order of 1000.degree. F. or more, which exacerbates the fouling and corrosion problem. This problem becomes particularly severe when the furnace temperature exceeds 2200.degree. F. The maximum service temperatures of common high temperature alloys are less than 2200.degree. F. for fuel-fired furnace atmospheres. Some noble metals such as platinum can withstand higher temperatures, but the cost becomes excessive.
One way of dealing with this problem has been to provide a large amount of water cooling to the nozzle so as to prevent high temperature corrosion or melting. However, a water cooling system is complex to operate and does not address the fouling problem where the furnace atmosphere has a high particulate content. Moreover, water cooling can escalate the corrosion and fouling problems when the furnace atmosphere contains condensible vapors.
Ceramic lances have been proposed as a solution to the fouling problem in high velocity gas injection. However, presently available ceramic lances are not suitable for industrial scale operations because of corrosion and cracking due to thermal and other stresses.
It is known that temperature effects on a nozzle may be ameliorated by recessing the nozzle in a cavity communicating with a combustion zone. However, a relatively large recess is required to achieve a significant beneficial effect. With high velocity gas injection, such a large recess may be detrimental because a large amount of corrosive furnace gas may be drawn into the cavity. Furthermore, this results in a reduction in the gas jet velocity. Thus, while the nozzle avoids temperature induced damage, this is offset by increased damage caused by contact with corrosive furnace gas drawn into the cavity.
Accordingly, it is an object of this invention to provide a lance which may be employed in a high velocity gas injection system and which will enable effective gas injection with reduced damage caused by high temperatures and corrosive gases.